The Catalog of Copyright Entries, published by the US Copyright Office,
contains a list of all copyright registrations received.
This information can be used to

See whether a copyright has been registered or renewed. This can be
useful for determining whether a work published after 1922 is now in
the public domain. For instance, copyrights
for works first published before 1964, and first published in the US,
that were not registered
and renewed in a timely manner, have now expired into the public domain.
(Some material that was first published abroad may be exempt from renewal
requirements.) To learn more about how to investigate the copyright renewal
status of a book, see this file.

Find out who registered a copyright, and what the copyright covers.
This can be useful if you want to contact a copyright owner to ask
permission to put an old work online.

This page includes pointers to electronic copies of copyright registrations.
We're particularly interested in renewal registrations to help find
public domain works, but we also collect original registrations.
We'd like to get help from the community
in putting this material online, and in doing
useful analyses of it.

Summaries and indexes

At present, there is no single searchable index for all online copyright
registrations. Below we list sites that allow a large selection of records
to be searched. We list them roughly in chronological order of
coverage.

The Copyright Office now has
Catalog of Copyright Entries and Catalogue of Title Entries volumes posted at the Internet Archive. Coverage: As of April 2013, all volumes of the run from 1891-1978 are included. Our lookup pages by year have alternate sources in the small number of cases where a volume that has in-force copyrights is missing from the Copyright Office scans.

Google Books's
Search of Scans of the Catalog of Copyright Entries offers full-text search, and links to individual volumes, of 91 volumes of copyright registrations from 1922 to 1977. Coverage: Original and renewal registrations for books, pamphlets, and contributions to periodicals, 1922-1977 (except for 1952, when only book registrations and renewals are available).

Philip Harper's site also had a list of copyright renewal transcriptions, sorted by year of original registrations. It's also identified some books that do not have renewed copyrights. Coverage: Limited; mostly 1950s renewals for books originally registered 1923-1928. The lists of selected books that are probably in the US public domain goes out further; but copyright status of all such books should be verified.

A list of first copyright renewals for periodicals is also available at this site. Coverage: Periodicals renewed from 1950 onward (for issues from 1923 onward); only the first renewal of each periodical is shown. Periodicals that first filed a renewal after 1977, or that did not renew copyrights, might not be included. I gave a talk on this resource at DLF in Spring 2006. (See Powerpoint slides or web conversion -- some images might not show up in the latter.)

The Copyright Office database is the definitive record of recent registrations. Coverage: All registrations (including renewals) from 1978 onward (except for a few 1978 registrations that did not make it into the database.)

Copyright records, by year

1891 - 1922

1923 - 1949

See this page
for original and renewal copyright records from 1923 to 1949.
Original copyrights from these years may still be in force;
renewal copyrights from these years (for original copyrights
from before 1923) have now expired in the US.

1950 - 1977 (with renewals of copyrights from 1922 - 1950)

These years are not yet in an official database; however, various projects
have scanned original and renewal copyright registration page images.
See also the transcriptions and indexes above.
Books may be covered under copyrights in related categories.
(For instance, if a portion of a book first appeared in a magazine, it
may still be copyrighted under the magazine's copyright even if the
book's copyright was not renewed.)
A few renewals for previous years were not cataloged until the 1978
issue; relevant page images for books and periodicals are linked to below.

The page images for original and renewal registrations can be found here:

1978 - present (with renewals of copyrights from 1950 - 1977)

You can see copyright registration records from 1978 onward from the
Copyright Office's
Copyright Records web site.
This will include renewals for copyrights from 1951 onward (and some, but
not all, renewals for 1950 copyrights),
as well as original registrations from 1978
onward.
For information on how to use the system, see
this file.
Note that due to changes in copyright law, all works copyrighted in 1964
or later automatically had their copyrights renewed, whether or not
a renewal was filed, and works copyrighted after 1977 have a single long term
with no renewal.

Credits

This work is supported by the University of Pennsylvania Libraries,
which hosts this site, supports its editor, and has scanned many of
the drama and image renewals,
by the Universal
Library Project
at Carnegie Mellon, which scanned page images for the "books" portion
of the
renewal volumes,
the Carnegie Mellon University Libraries,
which provide much of the storage space for the scanned images,
by the Carnegie Library of Pittsburgh,
the Free Library of Philadelphia,
the Newark Public Library, and
Stanford University Libraries,
which have supplied volumes to be scanned,
by Google Books, which has
scanned many of the Catalog of Copyright Entries volumes,
by Philip Harper and Project Gutenberg, who are providing
transcriptions of some of these records,
and by volunteers and donors
from the Internet community.
Volunteer scanners of renewal pages include Dianne Bean, Mary Mark Ockerbloom,
John Mark Ockerbloom, Juliet Sutherland, and Greg Weeks.
If you'd like to help out, write
onlinebooks@pobox.upenn.edu to find out more how you can get involved
with this project. Thanks to everyone helping out. (Thanks also
to the Copyright Office for compiling this material in the first place,
and for maintaining the online records from 1978 onward.)